Use over the recommended dose of about 120 μg can cause muscle tremors, headache, dizziness, and gastric irritation. Persons self-administering the drug for weight loss or to improve athletic performance have experienced nausea, vomiting, diaphoresis, palpitations, tachycardia, and myocardial infarction. Use of the drug may be confirmed by detecting its presence in semen or urine.[7]

Clenbuterol is not an ingredient of any therapeutic drug approved by the US Food and Drug Administration[citation needed] and is now banned for IOC-tested athletes.[8] In the US, administration of clenbuterol to any animal that could be used as food for human consumption is banned by the FDA.[9][10]

Although often used by bodybuilders during their "cutting" cycles,[11] the drug has been more recently known to the mainstream, particularly through publicized stories of use by celebrities such as Victoria Beckham,[8]Britney Spears, and Lindsay Lohan,
[12] for its off-label use as a weight-loss drug similar to usage of other sympathomimetic amines such as ephedrine, despite the lack of sufficient clinical testing either supporting or negating such use.

In 2006, San Francisco Giants pitcher Guillermo Mota, while a member of the New York Mets, received a 50-game suspension after testing positive for clenbuterol.[15] In 2012, MLB officials announced they were again suspending Mota for 100 games due a positive test for clenbuterol.[16]

In 2010, St. Louis Cardinals minor-league shortstop Lainer Bueno received a 50-game suspension for the 2011 season as a result of testing positive for clenbuterol.[19]

Cyclist Alberto Contador of Spain was banned for two years from professional cycling after testing positive for the drug at the 2010 Tour de France.[20] He was later stripped of the 2010 title of the Tour de France and the 2011 title of the Giro d'Italia.[21]CAS found that Contador probably tested positive due to a contaminated food supplement.[22] In 2013, Contador's teammate on the Team Saxo Bank squad, Michael Rogers, tested positive for clenbuterol at the Japan Cup bike race. In April 2014, the Union Cycliste Internationale announced that it accepted Rogers' explanation that the substance had been ingested by him after consuming contaminated meat whilst competing at the 2013 Tour of Beijing, upholding Rogers' disqualification from the Japan Cup, but declining to impose any further sanctions on him.[23]

In 2011, players of the Mexico national football team were found with clenbuterol in their bloodstreams, but were acquitted by WADA after they claimed the clenbuterol came from contaminated food. FIFA has also claimed 109 players from multiple countries
who were participating in the Under-17 World Cup in Mexico tested positive for this drug. However, FIFA and the World Anti-Doping Agency declined to prosecute any cases because the weight of evidence pointed to contamination from Mexican meat.[24][25]

In 2013, Mexican boxer Erik Morales was suspended for two years after testing positive for clenbuterol.[26]

In 2014, Toronto Maple Leafs Forward Carter Ashton was suspended from the NHL for 20 games without pay for violating the NHL/NHL Players' Association Performance Enhancing Substances Program after it was determined that he had ingested clenbuterol. Carter claimed he used an unprescribed asthma inhaler.[27]

In 2014, South Korean swimmer Kim Ji-heun has tested positive for clenbuterol at an out-of-competition test on May 13, 2014. After completion of proceedings by the Korea Anti-Doping Disciplinary Panel, Kim received a two-year suspension, back-dated to the day of his positive test.[28]

In 2015, two players from the Collingwood Football Club in Australia were delisted from the club and accepted two-year bans from all sport in Australia after testing positive to the substance, which they believe may have been in a contaminated illicit drug they consumed.[31][32]

In 2016, Raul A. Mondesi tested positive for clenbuterol and was suspended 50 games from the Northwest Arkansas Naturals. MLB and the MLBPA agreed to reduce the suspension from 80 games to 50 games after Mondesi claimed it was found in cold medicine.[35]

In 2016, the California State Athletic Commission decided to issue Francisco Vargas a temporary boxing license on a probationary basis after he tested positive for clenbuterol.[36]

In 2018, British Olympic sprinter Nigel Levine was provisionally suspended for failing a drug test.[37]

In March 2018, champion boxer Canelo Alvarez tested positive for clenbuterol on two consecutive urine tests in February 2018.[38] This violation led to the cancellation of a heavily anticipated rematch between Alvarez and Gennady Golovkin, which was to be held on May 5, 2018. Alvarez blamed the positive tests on consumption of contaminated meats from Mexico. Notably, some Mexican beef farmers are known to add clenbuterol to cattle feed.[39]

Intended to result in leaner meat with a higher muscle-to-fat ratio, the use of clenbuterol has been banned in meat since 1991 in the US and since 1996 in the European Union. The drug is banned due to health concerns about symptoms noted in consumers. These include increased heart rate, muscular tremors, headaches, nausea, fever, and chills. In several cases in Europe, these adverse symptoms have been temporary.[41]

Clenbuterol is a growth-promoting drug in the β agonist class of compounds. It is not licensed for use in China,[42] the United States,[41] or the EU[43] for food-producing animals, but some countries have approved it for animals not used for food, and a few countries have approved it for therapeutic uses in food-producing animals.

Not just athletes are affected by contamination. In Portugal, 50 people were reported as affected by clenbuterol in liver and pork between 1998 and 2002, while in 1990, veal liver was suspected of causing clenbuterol poisoning in 22 people in France and 135 people in Spain.[44]

In February 2009, at least 70 people in one Chinese province (Guangdong) suffered food poisoning after eating pig organs believed to contain clenbuterol residue. The victims complained of stomachaches and diarrhea after eating pig organs bought in local markets.[46][47]

In March 2011, China's Ministry of Agriculture said the government would launch a one-year crackdown on illegal additives in pig feed, after a subsidiary of Shuanghui Group, China's largest meat producer, was exposed for using clenbuterol-contaminated pork in its meat products. A total of 72 people in central Henan Province, where Shuanghui is based, were taken into police custody for allegedly producing, selling, or using clenbuterol.[48] The situation has dramatically improved in China since September 2011, when a ban of clenbuterol was announced by China's Ministry of Agriculture.[49]

Authorities around the world appear to be issuing stricter food safety requirements, such as the Food Safety Modernization Act in the United States, Canada's revision of their import regulations, China's new food laws published since 2009, South Africa's new food law, and many more global changes and restrictions.